Recently a digital image technology has been developed as the way of compressing video data efficiently because a digital image data, which is used in a digital cinema, a high-resolution television, a VOD (video on demand) receiver, a personal computer supporting MPEG (Moving Picture Experts Group) images, a video game console, a digital broadcasting receiver, and a cable television and the like, sharply increases during a digitalization of analog signals. Especially, many video compression standards including a MPEG-4 Part 10 AVC/H.264 reduce total data for transmitting video and increase a transmission rate by compressing to encode the video data. These standards reduce a temporal redundancy by using a correlation between continuous frames, and generally compress to encode the video by using a motion estimation technology and a motion compensation technology. That is, many video compression standards including the MPEG-4 Part 10 AVC/H.264 divide a present image into a block having a predetermined size; detect a most matching block with the present image block within a detection area of a previous image; and encode a motion vector which is difference components between the blocks. For example, the video encoder conform to the MPEG-4 Part 10 AVC/H.264 uses a video compression method per block unit which performs calculations by macroblock unit.
Meanwhile, depending on a motion degree, within a single frame, a detail or complexity degree of an object expressed by an image, and an easiness of prediction, the amount of residue data for each block which is the difference between a predicted image and an original image varies and a bit rate for encoding the residue data also varies. For example, the video encoder generates more bits for an input image having a high motion, detail, or complexity degree, and generates fewer bits for an input image having a low motion, detail, or complexity degree. The video encoder performs rate control to change the bit rate generated for each block in this manner depending on the frame. This rate control is for obtaining the optimum rate-distortion by assigning the bits properly for each frame and each block with respect to a predetermined objective bit rate. In this manner, the video encoder performs rate control to change a bit rate generated for each block in accordance with the frame. The rate control is for obtaining the optimum RD (Rate-Distortion) by assigning the bit for each frame and block with respect to a predetermined objective bit rate. The rate control can be performed such that the bit rate is adjusted by varying the scaling value used for quantization of a transformation coefficient which is obtained by performing a transformation such as a DCT (Discrete Cosine Transform) to the residue data. Here, the scaling value is a quantization parameter. To the block necessary for more bits due to the large residue, the lower quantization parameter is used for encoding the more data. And to the block necessary for fewer bits due to the small residue, the higher quantization parameter is used for encoding the less data.
In case of the MPEG-4 Part 10 AVC/H.264, there are eight different block modes to obtain an efficient compression and a high-definition simultaneously. Among each mode, the mode in which an objective function based on a RDO (Rate Distortion Optimization) has minimum value is selected. However, the amount of calculation is highly increased because various calculations for encoding to each mode are performed independently to select the optimum mode among eight different block modes, and the optimum mode needs to be selected again according to the newly selected quantization parameter in case that the bit rate is adjusted per block to obtain the optimum result by balancing the picture quality improvement and the bit rate reducing. That is, if various encoding calculation including the transformation such as the DCT process, quantization process and the like are used for rate control, the amount of calculation for real-time video encoding is rapidly increases. Accordingly, the efficient rate control for block unit is required for the rapid video encoder by reducing the calculation time and for high definition simultaneously.